Chromatin structure and architecture. DNA within the cell nucleus is packaged into chromatin and a variety of models presently describe the structure of the condensed 30 nm chromatin fiber. We are specifically interested in an understanding of the topological organization of DNA within condensed 30 nm chromatin fragments. In order to do this we are currently developing high resolution chromosome capture conformation assays utilizing both in vitro model systems, as well as native chromatin fragments, such as the previously studied condensed heterochromatin flanked by the developmentally regulated folate receptor and beta-globin genes. These studies will allow us to determine the structure of the 30 nm chromatin fiber, which will in turn provide a better understanding of the relations between chromatin structure and essential processes such as gene expression and DNA replication. Macromolecular assemblies. In collaboration with members of the Laboratory of Molecular Biology, and others, protein and protein-nucleic acid assemblies have been characterized in terms of their shape, stoichiometry and affinity of interaction using hydrodynamic methods. These studies extend current biochemical and structural investigations as exemplified by recently published studies on the monovalent lectin microvirin carried out in collaboration with Drs. G. Marius Clore and Carole Bewley. In this study, purified microvirin was shown to exist as a monodisperse monomer which binds monovalently to mannose alpha(1-2) mannose with an affinity of 50 micromolar. As this disaccharide unit terminates the arms of high mannose N-linked carbohydrate chains, microvirin is able to interact with the highly glycosylated HIV-1 surface protein gp120 and prevent HIV-1 entry into cultured cells. These biophysical and structural studies illustrate the molecular mechanism through which microvirin inhibits HIV entry (Shahzad-ul-Hussan et al., 2011).